text: In 2006 the strong violation of McCumber relation was observed for  Yb:GdSiO and reported in 3 independent journals. Typical behavior of the cross-sections reported is shown in Fig.2 with thick curves. The emission cross-section is practically zero at wavelength
  975 nm; this property makes  Yb:GdSiO an excellent material for efficient solid-state lasers.
  However, the property reported (thick curves) is not compatible with the second law of thermodynamics.  With such a material, the perpetual motion device would be possible. It would be sufficient to fill a box with reflecting walls with  Yb:GdSiO and allow it to exchange radiation with a black body through a spectrally-selective window which is transparent in vicinity of 975 nm and reflective at other wavelengths. Due to the lack of emissivity at 975 nm the medium should warm, breaking the thermal equilibrium.
  On the base of the second Law of thermodynamics, the experimental results 
   were refuted in 2007. With the McCumber theory, the correction was suggested for the effective emission cross section (black thin curve).
  Then this correction was confirmed experimentally.